Method for thawing frozen food

ABSTRACT

3. THE FOOD IS THEN HEATED TO THE SERVING TEMPERATURE BY THE SENSIBLE HEAT OF THE VAPOR AERIFORM GAS WHEREIN THE CONDENSATE IS REMOVED BY VAPORIZATION.   THE METHOD OF THAWING FROZEN FOOD WITHOUT COOKING BY PRODUCING A HOMOGENEOUS MIXTURE OF STEAM AND AIR WITH A DEWPOINT OF 170*F. OR LOWER AND A MIXTURE TEMPERATURE BELOW 212*F., EXTERNAL TO A THAWING CHAMBER, AND FORCING THE STEAM-AIR MIXTURE TO FLOW TOWARDS THE TOP AND BOTTOM SURFACES OF FROZEN FOOD SUPPORTED IN THE THAWING CHAMBER. THE METHOD OF RECONSTITUTING FROZEN FOOD IN THREE DISTINCTLY DIFFERENT STEPS: 1. A HOT GAS HEATS A FOOD CHAMBER TO THE SERVING TEMPERATURE OF FOOD. THE FOOD CHAMBER HAS A MULTIPLICITY OF APERTURRES POSITIONED IN THE TOP AND BOTTOM WALLS OF THE CHAMBER. 2. WATER IS VAPORIZED BY ADIABIATIC TRANFORMATION OF A PORTION OF THE SENSIBLE HEAT IN THE HOT GAS TO THE LATENT HEAT OF THE WATER VAPOR. THE WATER VAPOR IS THEN INTERMIXED EXTERNALLY TO THE FOOD CHAMBER WITH THE HOT GAS IN ORDER TO PRODUCE A VAPOR AERIFORM GAS WHICH HAS A TEMPERATURE SUBSTANTIALLY HIGHER THAN ITS DEWPOINT. THE VAPOR AERIFORM GAS IS FORCED INTO THE CHAMBER APERTURES IN ORDER TO CREATE TURBULENT JETS WHICH FLOW NORMAL TO THE TOP AND BOTTOM SURFACES OF THE FROZEN FOOD POSITIONED WITHIN THE FOOD CHAMBER. THE FOOD IS HEATED TO THE DEWPOINT OF THE VAPOR AERIFORM GAS BY VAPOR CONDENSATION PRIMARILY, THEREBY LEAVING THE CONDENSATE ON THE FOOD SURFACE.

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I METHOD FOR THAWING FROZEN FOOD Filed June `l, 1972 v l'7 Sheets-Sheet 18 l2, 1974 E. w. BALLENTINE METHOD FOR THAWING FROZEN FOOD 17 Sheets-Shet l 7 Filed June l. 1972 wNN United States Patent l[Office 3,796,804 Patented Mar. 12, 1974 3,796,804 METHOD FOR THAWING FROZEN FOOD Earle W. Ballentine, 3641 Via Palomino, Palos Verdes Estates, Calif. 90275 Continuation-impart of abandoned application Ser. No.

29,369, Apr. 3, 1970, which is a division of application Ser. No. 553,065, May 26, 1966, now Patent No.

3,604,334. This application June 1, 1972, Ser. No.

Inf. cl. A231 3/36l 0 U.s. cl. 426--524 z2 claims ABSTRACT OF THE DISCLOSURE The method of thawing frozen food without cooking by producing a homogeneous mixture of steam and air with a dewpoint of 170 F. or lower and a mixture temperature below 212 F., external to a thawing chamber, and forcing the steam-air mixture to iiow towards the top and bottom surfaces of frozen food supported in the thawing chamber.

The method of reconstituting frozen food in three distinctly different steps:

l. A hot gas heats a food chamber to the serving temperature of food. The food chamber has a multiplicity of apertures positioned in the top and bottom walls of the chamber.

2. Water is vaporized by adiabiatic transformation of a portion of the sensible heat in the hot gas to the latent heat of the water vapor. The water vapor is then intermixed externally to the food chamber with the hot gas in order to produce a vapor aeriform gas which has a temperature substantially higher than its dewpoint. The vapor aeriform gas is forced into the chamber apertures in order to create turbulent jets which flow normal to the top and bottom surfaces of the frozen food positioned within the food chamber. The food is heated to the dewpoint of the vapor aeriform gas by vapor condensation primarily, thereby leaving the condensate on the food surface.

3. The food is then heated to the serving temperature by the sensible heat of the Vapor aeriform gas wherein the condensate is removed by vaporization.

This is a continuation-impart of application Ser. No. 29,369 led Apr. 3, 1970 by Earle W. Ballentine, now abandoned, which, in turn, was a division of application Ser. No. 553,065 led May 26, 1966, now Pat. 3,604,334, issued Sept. 14, 1971.

BACKGROUND OF THE INVENTION The original purpose of the invention was to thaw without cooking, raw frozen protein such as meat, iish and fowl.

The salient conditions for this fast thaw process are:

1. The food should have a slab-like form with a thickness which preferably does not exceed 1% inches.

2.- The frozen food should be supported horizontally in a chamber with openings in the top and bottom walls of the chamber in the form of apertures, deilectors, or other guidance means for directing the flow of a gas.

3. A homogeneous saturated mixture of water vapor and air should be produced external to the chamber with a mixture temperature and dewpoint in the range of 140 F. to 150 F., if highest food quality is required. For maximum speed of thawing with acceptable food quality, a mixture temperature in the range of 160 to 170 F. should be used.

4. The homogeneous mixture should be forced to flow turbulently through the openings in the chamber walls with components of the How which are normal to the top and bottom surfaces of the frozen food supported in the chamber.

The apparatus of the invention is unique in that it has three forms. The first form produces the saturated mixture required for thawing Without cooking frozen proteins such as steaks, chops, chicken, etc. This form employs an enclosure with at least one opening for controlling the temperature of the aeriform gases which are mixed with the steam. The position of the opening is remote from the source of the hot gases. A second form of the apparatus also employs an enclosure with an opening and the position of this opening is adjacent to the source of hot gases. The third form of the apparatus does not employ an enclosure.

r[he reconstitution of frozen cooked foods requires the use of either the second or third forms of the apparatus. The salient conditions for the reconstitution process require in the first place the conditions for the fast thaw process stated above with the exception of condition 3. The need for preheating the thawing chamber which is desirable in the case of the fast thaw process is mandatory for the reconstitution process.

The reconstitution of frozen food such as a TV dinner is best accomplished by using a homogeneous mixture of water vapor and an aeriform gas wherein the preferred mixture temperature has a range of 350 to 425 F. and the preferred mixture dewpoint has a range of F. to F.

The new principle convective system preferably employs 8 to l5 turbulent jets impinging on the top and bottom surfaces of domestic size food portions.

The speed of reconstitution is ii-ve times as fast as a domestic kitchen range oven with four TV dinners.

A first object of the invention is to thaw, without cooking, frozen proteins such as steak, chops, chicken, etc.

A second object is to reconstitute frozen prepared food.

The fundamental and unique feature of the present invention is heat transfer from a homogeneous superheated mixture which is substantially higher than the mixture dewpoint to a food when the food temperature is above the dewpoint of the gaseous mixture. The homogeneous mixture will be defined to a vapor aeriform gas. An aeriform gas is usually air or a mixture of air with gas burner products.

A major feature of the invention is a radically new principle forced convection system which provides a serving temperature distribution throughout a homogeneous food with a temperature variance of only a few degrees.

This invention also teaches the production of a Ivapor aeriform gas by transforming a portion of the sensible heat of a gas flame into the latent heat of water vapor, and then mixing the gas products with the Water vapor.

Another object is to reduce the loss of moisture from the food during the reconstitution process.

A further object is to prevent charring or discoloration of the food during reconstitution.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, its objects and features and further objects, features, and advantages thereof will be better understood from the following description, which merely illustrates exemplary preferred embodiments of structure which may be utilized to practice the invention taken in connection with the accompanying drawings, in which:

FIG. 1 illustrates two measured curves, respectively showing the development of thermal energy by condensation as a function of surface temperature and for two different mixtures of air and steam;

FIG. 2 illustrates, as a function of time, the surface temperature and the chamber of temperature as measured when the thawing chamber is charged with an air-steam mixture having 159 F. prior to injection into the thawing chamber;

FIG. 3 illustrates two measured curves, both plotted as functions of temperature, one curve showing the mixing ratio of air and steam for the several temperatures, the other one sho-wing pressure in the boiler developing the steam for the mixture;

FIGS. 4a to 4d show schematically the progress of thawing from the surface of a piece of meat into the interior thereof;

FIG. 5 shows several measured curves representing temperature as a function of time and having depths from the surface into a piece of originally frozen meat, as parameter;

FIG. 6a illustrates actual thawing time and total thawing time as a function of meat thickness, the measured curves differing by non-inclusion or inclusion of the time for preheating the exact amount of water to be evaporated for completing thawing;

FIG. 6b shows a comparison of empirical thawing time with the theoretical law of times which is derived from the Fourier equation;

FIG. 7 illustrates two measured curves, respectively representing pressure in a thawing chamber and rate of ow of steam and air as a function of various equivalent standard size orifices approximately representing different venting outlet sizes for a thawing chamber charged with a steam-air mixture;

FIG. `8 displays a relation between the heat transfer coefficient of a turbulent jet and the static pressure which produces the ow;

FIG. 9 shows the center line velocity and the width of a turbulent jet as a function of the distance downstream from the jet orifice;

FIG. 10 shows the relationship between the relative heat transfer coefficient and the number of turbulent jet orifices;

FIG. 1l compares the heat transfer coefficients of a condensing vapor with those of a turbulent jet and forced convection of a permanent gas;

FIG. 12 shows a derived empirical curve which gives the dew point of a vapor aeriform gas as a function of its composition;

FIG. 13 presents three curves, the first of which relates to tbe loss of moisture by food at various temperatures in dry air, the other two derived curves relating to the gain or loss of moisture by food to vapor aeriform gases with two different dew points as a function of food temperature;

FIG. 14 graphically displays the conditions for adiabatically transforming a gas flame into vapor aeriform gases with different temperatures Iand dew points;

FIGS. 15 Vand 16 illustrate fast thaw devices for home use;

FIG. 17 illustrates an elevation of the salient components of a form of apparatus for reconstituting frozen food which can be incorporated in a domestic gas range;

FIG. 18 is a top View of the apparatus; l

FIG. 19 illustrates the salient components of a form of apparatus for the large scale reconstitution of frozen food.

The curves plotted in the several figures will aid in the detailed description of my solution to the problem which is, how to apply a suflicient amount of thermal energy to a frozen piece of food without cooking it, and in a rather short period of time. From a physics standpoint, this problem presents itself in this manner. The frozen piece of food has a surface which is an interface of a solid body with the environment. From a point in the environment remote from this interface heat has to be supplied to a point in the interior of the piece, which has to be regarded as being also remote from the interface. Hence, one can distinguish between three phases of heat transport: (l) from the remote point in the environment (or heat source) to a point in the vicinity of the surface; (2) from that latter point into the interface or surface of the piece of food, and (3) from the latter surface region into the interior of the piece of food including the center thereof as the area remotest from the surface. The interior can be regarded as a heat sink for the thawing process itself. In the following these three phases shall be discussed in detail.

It is apparent that one has control over the last phase or process only to the extent of influencing the surface temperature. The heat conduction mechanism itself as it takes place in the interior of the piece of food is determined solely by the physical characteristics of the food. The situation is entirely different for the transport of heat through the environment (Phase l) and into the surface of the piece of food to be thawed (Phase 2). Here now we turn to the particular condition set, namely, that the transport of heat to the surface must not provide a surface temperature at which the piece of food will cook. Accordingly, the heat supply process must be such that the temperature of meat, for example, remains below 170 F. for thawing periods of, say, 15 to 20 minutes or less.

It is apparent that the heat conduction Phase 2, i.e., the supply of heat onto and into the surface of the frozen piece sets the critical operating condition; the development of thermal energy in or on the surface will determine the surface temperature of the food and the food must not cook as long as the heat transport continues. Particlarly, it is essential that the entire amount of heat needed to thaw the entire piece of food be applied to the surface of meat without any cooking being accomplished. Therefore, Phase 2 shall be described first.

For this Phase 2, I have chosen to use the phenomena of condensation, as this is by far the fastest mechanism to transport heat from a point in the exterior of the surface onto the surface itself. A condensate has, at the instant immediately succeeding condensation, the same temperature the vapor had at the instant prior to condensation. The heat content of the vapor occupying a rather large volume prior to condensation, is directly and suddenly available on the surface as precipitation. The condensate occupies only a very small volume, permitting rapid transfer of the same heat content from the small amount of condensate to the cooler surface itself. Thermal energy content in a given volume of steam adjacent to the Surface is thus transferred from the environment at the speed of the volume contraction due to the condensation onto the particular surface on which the condensate is formed. This is a heat transport resembling an implosion, and is far superior to any conduction by any kind of molecular transport mechanism of thermal energy.

The vapor to be brought into engagement ultimately determines the surface temperature of the surface of the object on which the condensate develops. The vapor, when coming into contact with the frozen piece of food should therefore have a temperature below a short term cooking temperature, as the surface of the food will tend to assume the temperature of the condensate.

It follows from the foregoing, that the vapor as brought into the vicinity of the food to be thawed should yield maximum possible condensation at a long term cooking temperature. This is obtained by selecting the vapor pressure so that for the particular operating temperature the vapor can be at equilibrium with the liquid phase of similar temperature. As long as the surface on which the condensate is formed is colder than the temperature, there will be immediate heat exchange between the condensate and the surface. The condensate is cooled so that the vapor contacts the liquid phase at a temperature lower than that required for equilibrium. Thus, condensation will continue as long as the surface is cooler.

Due to its high latent heat of vaporization, water vapor, i.e., steam is most suited. For operating temperatures as envisioned here, F. to 170 F., the water vapor pressure is below atmospheric pressure for equilibrium conditions with the liquid phase. However, the thawing process cannot be carried out in a below atmospheric pressure environment because of venting requirements and because ow of steam is needed to replenish the precipitated vapor. Thus, the steam must be mixed with a gas.

A steam-air mixture having a temperature such that the, steam, at its partial pressure, is in equilibrium with water of like temperature, is a steam saturated mixture. The mixture of air and .steam used thus has a dew point which constitutes the operating temperature. The dew point is a very unique temperature in the case of a saturated mixture of steam and air. When frozen food is thawed by a saturated steam-air mixture, the temperature of the food will rise rapidly until it reaches a value which is about to 15 F. below the dew point. FIG. 1 shows that the transport of heat from the steam mixture to the food is more or less constant until the food reaches a temperature of about 155 F. when the steam-air mixture has a dew point of 168 F. The food temperature cannot exceed the ldew point, of course, because the temperature gradient between the steam-air mixture and the food would be zero. This unique property of a saturated steamair mixture makes it possible to thaw frozen uncooked food without cooking by choosing a dew point which is below the cooking temperature for the particular food. (Saturated mixtures with a temperature and dew point above the boiling point of water are used for fast thawing of frozen cooked vegetables in commercial kitchens.) There is one disadvantage of the saturated mixture, namely, its low temperature.

The transport of heat from a steam-air mixture to the frozen food depends on the difference in temperature between the steam-air mixture and the frozen food. The temperatures which do not cook food are relatively low and the rate of transport of heat from the steam-air mixture to the frozen food is correspondingly slow. For this reason, a compromise must be effected in choosing the dew point, which means that the temperature of the steam-air mixture must be as high as possible as determined by the appearance of the food after thawing. By using a higher dew point the transport of heat increases in two ways, namely, the larger steam-air to food temperature gradient rand the larger amount of latent heat released when the water vapor in the mixture condenses on the food. It is of interest to consider the magnitude of the amount of latent heat in two saturated steam-air mixtures with dew points of 140 F. and 160 F. The dew point is determined by the vapor pressure of water at the deW point which, in the case of a saturated mixture, is the mixture temperature and the molecular weight of the permanent gas, which in this case is air. The well-known steam tables show that water at 140 F. has a vapor pressure of about 2.9 lbs./in.2. This pressure is, therefore, Daltons partial vapor pressure of the water vapor. The partial pressure of air is, of course, the difference between the atmospheric pressure of 14.7 1bs./in.2 and the partial water vapor pressure of 2.9 lbs./in.2, namely, 11.8 1bs./ in?. The molecular weights of air and water vapor are 29 and 18, respectively. The composition of the steam-air mixture is obtained by multiplying the ratio of the partial pressures by the ratio of the molecular weight ratios, i.e.,

of air per pound of steam.

FIG. 2 gives the composition of steam-air mixture for dew points of 155 F. to 168 F. and FIG. 12 gives the composition of steam-air mixtures for dew points of 110 F. to 170 F. 'In order to determine the amount of latent heat in the two steam-air mixtures, it is necessary to know the value of the latent heat of steam. Since the latent heat of steam depends on its pressure we will assume a latent heat of 1000 B.t.u./lb. Thus, steam-air mixture having a weight of 7.5 lbs. has a latent heat of 1000 B.t.u., which means that one pound of the steam-air mixture has a latent heat of 1000/7.5:133 B.t.u./lb.

The vapor pressure of water at a temperature of 160 F. is 4.7 lbs./in.2 so that the composition of a steam-air mixture with a dew point of 160 F. is 4.8 lbs. of air to 1.0 lbs. of steam. Thus, the 160 F. dew point has a latent heat of 208 B.t.u./lb. The sensible heat of these steam-air mixtures is small compared to their latent heats. The frozen food comes out of the freezer at a temperature of about 0 F. The specific heat of air is 0.24 B.t.u./ lb. F., and the specific heat of steam is 0.47 B.t.u./lb./ F. The sensible heat is the product of specific heat temperature rise and mass. Thus, the sensible heat content of one pound of F. dew point steam-air mixture is 38 B.t.u. compared to 133 B.t.u. of latent heat. The sensible heat content of the 160 F. dew point mixture is 43 B.t.u. The effect of increasing the dew point from 140 F. to 160 F. is to increase the enthalpy of the mixture from 171 B.tu./lb. to 251 B.t.u./lb. and to increase the temperature gradient between the steam-air mixture and the frozen food initial temperature by 20 F. This larger temperature gradient would be most important when the food temperature approaches its nal value. If an oven at a temperature of 160 F. is used to thaw frozen food, one pound of the oven air at 160 F. would have an enthalpy above 0 F. of 38.5 B.t.u., which is to be compared with an enthalpy of 251 B.t.u. for a pound of the steam-air mixture which is a ratio of 6.5 to 1. Thus, it is found that the dew point of a saturated steam-air mixture and a means for controlling it is of paramount importance in the fast thawing of frozen uncooked food. without cooking, discoloration, or loss of natural juices. The dew point and operating temperature is selected in the range of long term cooking temperatures, below the short term cooking temperature of, for example, meat. The definition of short term thus includes the approximate period of time needed for maintaining this condensation process for completion of thawing, and it is basic to the discovery I made, that this steam process of applying thermal energy to the frozen food needs to last only for so short a period that no cooking will ensue.

Steam Iwill precipitate by condensation as long as a mixture of air and steam at saturation contacts the surface of the food and that the food temperature is below the dew point. The surface thereof will heat up faster than obtainable with any other type of conduction while it is ensured that the surface of the food will not cook if the flow of steam is terminated sufliciently early.

An operating temperature and dew point of 140 F. to F. of a mixture of steam and air as it ows around frozen meat during thawing is most suitable and can be obtained by providing initially a steam-air mixture having about 4.8 to 6.5 pounds of air per each pound of steam. When such a mixture is brought into intimate contact with a piece of frozen meat, one obtains the fastest possible transfer of heat to the surface of the meat without cooking it.

One can see the following: A steam-air mixture having that operating temperature, but being not saturated, contains, therefore, less vapor, and less vapor per unit volume would be available for condensation so that the heat transfer is necessarily less than if saturated steam were used. An increase in the amount of steam available for condensation requires necessarily an increase of the temperature which is not desired, because cooking is to be avoided. Thus, saturated steam at a long term cooking temperature provides optimum conditions of heat transport according to Phase 2.

Without going into details at the moment as to quantitative details, FIG. 1 shows two curves, A and B, each representing the transfer of heat by means of condensation onto a surface at the several temperatures thereof as plotted along the abscssa. The two curves differ by a parameter which is the operating temperature and deW 

1. A HOT GAS HEATS A FOOD CHAMBER TO THE SERVING TEMPERATURE OF FOOD. THE FOOD CHAMBER HAS A MULTIPLICITY OF APERTURRES POSITIONED IN THE TOP AND BOTTOM WALLS OF THE CHAMBER.
 2. WATER IS VAPORIZED BY ADIABIATIC TRANFORMATION OF A PORTION OF THE SENSIBLE HEAT IN THE HOT GAS TO THE LATENT HEAT OF THE WATER VAPOR. THE WATER VAPOR IS THEN INTERMIXED EXTERNALLY TO THE FOOD CHAMBER WITH THE HOT GAS IN ORDER TO PRODUCE A VAPOR AERIFORM GAS WHICH HAS A TEMPERATURE SUBSTANTIALLY HIGHER THAN ITS DEWPOINT. THE VAPOR AERIFORM GAS IS FORCED INTO THE CHAMBER APERTURES IN ORDER TO CREATE TURBULENT JETS WHICH FLOW NORMAL TO THE TOP AND BOTTOM SURFACES OF THE FROZEN FOOD POSITIONED WITHIN THE FOOD CHAMBER. THE FOOD IS HEATED TO THE DEWPOINT OF THE VAPOR AERIFORM GAS BY VAPOR CONDENSATION PRIMARILY, THEREBY LEAVING THE CONDENSATE ON THE FOOD SURFACE.
 3. THE FOOD IS THEN HEATED TO THE SERVING TEMPERATURE BY THE SENSIBLE HEAT OF THE VAPOR AERIFORM GAS WHEREIN THE CONDENSATE IS REMOVED BY VAPORIZATION. 